As a source for Sodium, Water Softeners use Sodium Chloride, also known as Cooking Salt. Since Cooking Salt is a very cheap and common resource, it could be used in large quantities, without any considerable costs or efforts.

A Water Softener system includes a number of cylindrical tanks and a large squared container. The tanks store the already treated water (their number depends on the model and the level of water output required from the system). The container stores the salt the system uses during the Regeneration Process. The container is filled with salt by the users.

When put to domestic use, a Water Softener is installed on the main pipe entrance of the plumbing system into the house. This way all the water entering the indoor plumbing system are treated and Soft.
The most common models of domestic Water Softeners are:

Electric single-tank Softeners
Besides operating by electric power, the single-tank also poses a major flaw in their design. During a Regeneration Process, the water flow pressure drops dramatically. That is why single-tank Softeners tend to be timed to regenerate at night, when the water use is at its lowest rate. This is not much comfort, since during the rest of the day the single-tank may ran out of Soft water, not to mention that the Regeneration Process is quite noisy, fitting right in as a soothing bedtime story for your offsprings.

Hydraulic twin-tank softeners
Not only do they operate by the water flow alone, the twin-tank feature also enables them to supply Soft water all day long. While the Softener performs a Regeneration Process to one tank, the other will continue the regular supply of Soft water. The latter will in turn undergo a Regeneration of its own, while the first tank will supply Soft water, and vice versa.

A Water Softener can be a nuisance when it comes to maintaining its salt container. As said, the Softener has a salt container which is filled by its users. Salt should be added to the container on a periodic basis (a few weeks to months, depending on the specific water use), but since most Softeners do not include any indicator of the salt level, one may forget to refill the container. The system won't be impressed with an empty container, and will continue to operate as usual, contaminating the water tanks with Hard water.

From personal experience, I can say that most people gradually learn their water use habits, and add salt in accordance with them. Yet sometimes an unforeseen event, for example, an undetected pipe leak, may lead to a substantial increase in the water use, emptying the salt container prematurely, and leaving the system vulnerable.